Heretofore various systems have been studied and developed for the communication system. The spread spectrum communication system is known as one of the systems having a high reliability.
By this spread spectrum communication system, on the transmitter side, information is transmitted by making primary modulated signals such as narrow band data of base band information, sound, etc. hop to a plurality of frequencies in a wide band at a high speed (FH system, Frequency Hopping), or by spreading spectrum to a wide band by using a high speed pseudo noise code (PN code) (DS system, Direct Sequence), or by spreading spectrum by combining them (FH/DS system), and on the receiver side the wide band signals are inversely spread to the original narrow band primary modulated signal by means of a correlator to reproduce the information signals. Recently attention is paid to this spread spectrum communication system as a communication system having an extremely high reliability from view points that it is resistant against external interference and noise, that it has further a high secrecy, etc.
One of the most important points of this spread spectrum communication system is the construction of the correlator on the receiver side. A correlator, which is believed to be the simplest and the most convenient and to have a high reliability at present in the wireless spread spectrum communication, is a device using surface acoustic wave (hereinbelow abbreviated to SAW).
As an SAW correlator there are known in general the correlator (tapped delay line type) and the convolver type. Here, although the correlator type has a simple construction and generally a high efficiency, it is influenced seriously by the temperature coefficient of the substrate. On the other hand, although the convolver type is hardly influenced by temperature variations, the efficiency thereof is generally low. However, in the case where the PN code described above is variable, the correlator type can not deal therewith, because the PN code is fixed. On the contrary, the sort of the PN code can be varied for the convolver type. Consequently, if the efficiency is at a practically usable level, a correlator of convolver type is much more easily used.
Further, in the spread spectrum system, by the DS system, since a high speed PN code is mixed with base band information by means of one mixer to transform it into a wide band information, this system can be very simply realized. However it is weak in disturbance or separation from other channels, or in the distance problem.
The signal spread directly by the PN code is subjected to a correlation processing with a reference signal by means of a correlator in the IF stage on the receiver side. When the PN code of the receiver side is in accordance with that of the transmitter side, a correlation peak is outputted from the correlator. However, when the ratio of the total power of the spread spectrum signal to the total power of the spectrum of disturbance, etc. is close to or greater than the process gain (gain of processing) of the convolver, even in the spread spectrum communication, communication errors are produced only by the DS system.
FIGS. 7A, 7B and 7C as well as FIGS. 8A, 8B and 8C illustrate problematical points of the prior art system. These figures show an example, in which the modulation method of the system is so determined that, an SAW convolver being used as the correlator by the DS system, when the base band information is "1", a convolution peak is outputted and when the base band information is "0", no convolution peak is outputted.
As indicated in FIGS. 7A, 7B and 7C, when there are no disturbance, etc., the convoution peak is correctly restored, depending on whether the information is "1" or "0". However, when the ratio of the total power of the spread spectrum signal to the total power of the spectrum of disturbance, etc. is close to or greater than the process gain of the convolver, it is not possible to judge whether the information is "1" or "0" and therefore an error is produced.
Consequently, it is necessary to remove this disturbing wave, etc. to improve the anti-disturbance property by some other signal processing.
As one of the efficient improving methods it is useful to remove disturbing spectrum indicated in FIG. 8A by means of a filter. However, since it can not be known previously where disturbed spectrum is produced, a programmable filter working in real time is necessary. However no satisfactory filter of this kind has been developed yet.
As described above, since it can not be judged previously, at which frequency position of the desired spectrum in the spread spectrum communication disturbance, etc. appear, it is desired to improve the worsening in the S/N ratio due to disturbance, at whichever frequency position the disturbing wave appears.